1. Technical Field
The present invention relates to a cell selection system for selecting a cell to be used for communications by a mobile terminal supporting a plurality of communications protocols.
2. Background Art
One major objective of LTE (Long Term Evolution)/SAE (System Architecture Evolution) being standardized in 3GPP is to support a plurality of RATs (radio access technologies). There are many types of RATs, such as UTRAN, GERAN, E-UTRAN, WLAN 3GPP access system, and non-3GPP access system. Each of the plurality of RATs has different properties (coverage, transmission rate, cost of introduction and operation, etc.).
FIG. 14 shows an example of coverages of a plurality of RATs. It can be seen that each RAT has a different coverage. FIG. 14 shows that the coverage of a GPRS network, which supports: GSM (Global System for Mobile)/GPRS (General Packet Radio Service), which is a cellular system having been used for a long time in Europe, the United States, and the like; and UMTS (Universal Mobile Telecommunications System), which is newly used particularly in Japan and the like, is wide, that the coverage of an Evolved network, which has been newly introduced, is narrower, and that the coverage of a WLAN, which is used for a hot spot or the like, is further narrower. Each RAT also has a different transmission rate; for example, WLAN>E-UTRAN (Evolved UMTS Radio Access Network: a general term for wireless equipment on the network side in Evolved networks (including base stations))>UTRAN (UMTS Radio Access Network: a general term for wireless equipment on the network side of UMTS in GPRS networks (including base stations and RNCs (Radio Network Controllers))).
A technology for supporting a plurality of RATs is desired in order to effectively utilize such properties that differ from RAT to RAT. Supporting a plurality of RATs will provide, for example, a choice of using a UTRAN, a GPRS network having a wide coverage, when a terminal moves at high speed, using a WLAN when the frequency of movement is low but a high-speed communication is required, and the like.
In UMTS and LTE, which are specified by 3GPP, mobile control is performed in which a terminal determines a cell to connect to when the terminal is in an idle mode. For example, if a terminal can communicate with both a cell of a mobile phone network and a cell of a WLAN, the terminal chooses which cell to communicate through. The operation of a terminal selecting a cell is called “cell reselection.” Cell reselection is carried out by a terminal receiving broadcast information transmitted from a base station, and performing mobile control in accordance with cell reselection information included in the broadcast information. Since the selection of a cell is carried out in accordance with cell reselection information included in broadcast information, a base station can control the movement of a terminal with the cell reselection information that the base station includes in the broadcast information.
Currently, the operation of cell reselection in LTE is under discussion and remains undefined. The basic principle of the operation of reselection is expected to follow the operation in UMTS. The operation of cell reselection in UMTS will be described here.
Information related to cell reselection is transmitted mainly in SIB (System Information Block) 3 and SIB 4 of broadcast information. The difference between SIBs 3 and 4 here is that SIB 3 is used for both a terminal being in an idle mode and a terminal being in a connected mode whereas SIB 4 is used only for a terminal being in a connected mode. This is because, in UMTS, there is a terminal state where reselection is performed even in a connected mode (RRC states CELL_FACH, CELL_PCH, and URA_PCH). In LTE, however, cell reselection is not supposed to be performed outside an idle mode, and therefore only one SIB is used for transmission of cell reselection information. Information on a neighboring cell is notified of by SIBs 11 and 12. The relation between SIBs 11 and 12 is the same as that between SIBs 3 and 4.
Main parameters used for judgment about cell reselection will be described here. Main parameters include “Cell selection and reselection quality measure,” “Ssearch,” “Qrxlevmin/Qqualmin,” “Qhyst,” “Treselection,” “Scaling factor,” and “Cell Info.” Each parameter will be described next.
FIG. 15 illustrates parameters used for judgment about cell reselection. “Cell selection and reselection quality measure” is an instruction on a measuring method used for performing cell selection and cell reselection. In UMTS, it is a parameter to indicate CPICH Ec/No or CPICH RSCP.
“Ssearch” is a radio field intensity parameter to judge whether the quality of a currently selected (which is called “camp on” in 3GPP) cell is to be judged or not. If a terminal were to perform detection and monitoring of its neighboring cell all the time, the terminal would consume much battery. The quality of a currently selected cell is compared with the parameter “Ssearch,” and the detection and monitoring of the neighboring cell is not performed when the quality of the current cell is better than the parameter “Ssearch.” As shown in FIG. 15, if the radio field intensity in a currently selected cell (change source) is higher than the parameter “Ssearch,” another cell (change destination) is not searched for; and the search for another cell starts when the radio field intensity in the current cell becomes equal to or lower than the parameter “Ssearch” at Time T1.
“Qrxlevmin/Qqualmin” is a radio field intensity parameter to judge whether a detected cell should be monitored or not. A cell whose radio field intensity is lower than this parameter is judged not to require monitoring. As shown in FIG. 15, a cell to be monitored as a candidate change destination requires that the radio field intensity be equal to or higher than the parameter “Qrxlevmin.”
“Qhyst” is a radio field intensity parameter that is set in order to avoid a so-called “ping-pong state.” The “ping-pong state” is the following phenomenon. For example, when there is a terminal near the boundary of two cells, the intensities of radio waves received from the two cells may be almost equal. In this case, if a process were to be performed in which one cell is changed to the other whose radio field intensity is even a little higher than the one, the cell change might frequently occur. In order to prevent this phenomenon, the reception quality of a detected cell is compared with the value of the reception quality of a currently selected cell added with the parameter “Qhyst,” and the change is controlled not to be performed unless the reception quality of the detected cell is better than the reception quality of the currently selected cell by the parameter “Qhyst” or more.
“Treselection” is a time parameter to avoid a frequent occurrence of cell changes when there are significant fluctuations in the reception quality of a terminal. A terminal performs the cell change when a state, in which the reception quality of a detected cell is better than the value of the reception quality of a currently selected cell added with the above-mentioned parameter “Qhyst,” has continued for a time corresponding to the parameter “Treselection.” That is, as shown in FIG. 15, if the radio field intensity in a detected cell becomes greater than the value of the radio field intensity in a currently selected cell added with the parameter “Qhyst” at Time T2, the mobile terminal measures the time for which the state continues, and performs the cell change at Time T3 when the duration becomes equal to or greater than the parameter “Treselection.”
“Scaling factor” is a parameter to change the value of “Treselection” in accordance with the moving speed of a terminal. The value of “Treselection” is reduced for fast-moving terminals, since they require a rapid cell reselection. For that purpose, the value of “Treselection” is changed by means of the parameter “Scaling factor” if a terminal is judged to be moving at high speed. Whether a terminal is moving at high speed or not can be judged by the number of cell changes per unit time or the like.
“Cell Info” is a parameter to notify a terminal of information on another cell. The main pieces of information include: “Frequency,” which notifies of the frequency band of the other cell; “Scrambling code,” which indicates the code under which the other cell is being operated; and “Cell individual offset,” which notifies, by means of an offset, of the degree of ease with which the cell is selected. “Cell individual offset” is notified of in SIB 11.
As seen above, a detailed mobile control could be performed on a terminal being in an idle mode by transmitting various parameters in UMTS (LTE also is thought to be going to have an operation fairly close to that in UMTS).    Non-patent document 1: 3GPP TS 25.331 V7.4.0, “Radio Resource Control (RRC); Protocol specification”    Non-patent document 2: 3GPP TS 25.304 V7.2.0, “User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode”    Non-patent document 3: 3GPP TR 36.938 V0.4.0, “Improved network controlled mobility between E-UTRAN and 3GPP2/mobile WiMAX radio technologies”